As is well known, state-of-art warning horns are mostly built around a simple series connection of a coil and a breaker inside the warning horn. The breaker is controlled from the coil through a battery power supply. Warning horns of this kind produce electromagnetic emissions in considerable amounts. In fact, their working frequency in the power-on condition is about 400-500 Hz, and their sound emission is accompanied by electromagnetic emissions due to the interruption of inductive currents. The currents flowed through warning horns powered at 12 V may be as large as 8-9 Amperes.
There exists a European Community (E.C.) Standard 95/54 of Nov. 11, 1995 aimed at keeping electromagnetic emissions within bounds. Warning horns of the above type do not fall within the limits provided by that standard. Thus, there is a need for warning horns or horns which would conform with the restrictions enforced by the standards for electromagnetic emissions.
The state-of-the-art provides some approaches for implementing warning horn drivers. A first approach is shown in the accompanying FIG. 1, wherein the warning horn 2 is operated by means of a push-button 20, when small supply currents are provided. FIG. 2 shows a similar approach, wherein the warning horn 2 is operated through an electromagnetic or electronic relay 16, in the instance of large currents being supplied.
The horn operation is the same in either cases. Upon the supply current to the breaker 15 reaching its largest value, the normally closed breaker opens and interrupts an electric connection, at a working frequency equal to the resonance frequency of the device. This frequency can be calibrated to a selected value in the 400 to 500 Hz range. The breaker is expected to also cut off an inductive current of as much as 8 or 9 A dissipating an inductive energy of about 36 mJ per cycle. In power terms, the dissipation amounts to 14 to 18 W in the instance of a horn operated at 12 V.
As previously mentioned, the most serious aspect of the problem is related to the need for reducing the electromagnetic noise emission from the arcing produced during the interruptions of the inductive current flow, more so than to the dissipation of power.
FIG. 3 shows a third approach wherein a non-electrolytic capacitor is connected in parallel with the breaker. This capacitor may have a capacitance of a few microfarads, and can store up all of the inductive energy emitted as the breaker opens. Thus, the breaker will open with no arcing being produced, but a serious problem is encountered as the breaker then closes to connect the capacitor to the battery supply. These two components have radically different voltages, and when connected in parallel, cause oscillations with current spikes of up to 50 A. Accordingly, this third approach neither solves the problems created by electromagnetic disturbance nor those related to energy dissipation.
The state-of-the-art provides a fourth approach directed to reducing electromagnetic noise. This fourth approach comprises an electronic power switch driven from the breaker, according to the diagram of FIG. 4. While in many ways advantageous, not even this approach is devoid of problems, due to the large amount of power that must be dissipated through the driver circuit. In fact, the inductive energy will be discharged to the power switch, and the latter must be provided with a large-size dissipator complete with a voltage clamping device. In any case, this approach requires a dissipating element, represented here by the power switch coupled to a large-size dissipator.